You're not protecting your child and you're naive if you think the drugs won't come back.
People who lack resilience don't suddenly develop resilience with a stint of therapy. It takes a LOT of therapy and commitment and life altering to build resilience, but the next time something stressful happens, an argument, a bereavement, a disciplinary etc. The drugs will come back.
And if they do what do you think social services would say?
And if your first thought is that you'd do anything to not have them involved in your life, then the best way to do that is to ship this one back to his mum for good and let him be her problem.
Also, get some therapy for yourself. At the very least do Al Anon. Your mindset of you're going to give it another go, but if it doesn't work you can say you tried is deeply codependent, and selfish behaviour.
Your child has already had him walk out once, and now he will go again, either by his choice or your choice, and that is extremely damaging.
If you would do anything for your child, do this and get rid of this man.